1. Field of the Invention
The present invention relates to a bipolar transistor and, more particularly, to a polysilicon-edge, base-emitter super self-aligned, low-power, high-frequency bipolar transistor and a method of forming the transistor.
2. Description of the Related Art
A high-frequency bipolar transistor is a device that can turn off and on again fast enough to respond to a high-frequency signal without distorting the wave shape of the signal. A low-power high-frequency bipolar transistor is a device that consumes very little power in responding to the high-frequency signal. Low-power high-frequency bipolar transistors are used in wireless applications, and are finding uses in emerging optical networking applications.
FIG. 1 shows a cross-sectional diagram that illustrates a portion of a prior-art, low-power high-frequency bipolar transistor 100. As shown in FIG. 1, transistor 100 includes a collector layer 110, a base layer 112 that is formed on collector layer 110, and a field oxide region FOX that adjoins layer 112. In addition, transistor 100 includes a thin oxide layer 114 that is formed on a portion of base layer 112 and the field oxide region FOX, and a n+ extrinsic emitter 116 that is formed on thin oxide layer 114.
As further shown in FIG. 1, transistor 100 also includes an n+ emitter region 118 that is formed in base layer 112, and an n+ poly ridge 120 that is connected to extrinsic emitter 116 and n+ emitter region 118. Extrinsic emitter 116, emitter region 118, and poly ridge 120 form the emitter of the transistor.
Transistor 100 additionally includes a silicided base contact 122 that is formed on base layer 112, and a silicided emitter contact 124 that is formed on extrinsic emitter 116. In addition, an oxide spacer 126 is formed on base layer 112 between poly ridge 120 and base contact 122.
During fabrication, emitter region 118 is formed from dopants diffusing from poly ridge 120 into base layer 112. As a result, a very small base-to-emitter junction results. A small base-to-emitter junction reduces the base-to-emitter capacitance. Reduced capacitance, in turn, provides low-power high-frequency operation.
One drawback of transistor 100, however, is that transistor 100 has a large base-to-collector capacitance which, in turn, limits the operation of the transistor. Thus, there is a need for a low-power high-frequency bipolar transistor with a reduced base-to-emitter capacitance and base-to-collector capacitance.
The present invention provides a low-power high-frequency bipolar transistor that reduces the base resistance, the base-to-emitter capacitance, and the base-to-collector capacitance. The bipolar transistor of the present invention is formed on a wafer that has a buried layer and a first epitaxial layer of a first conductivity type. The first epitaxial layer is formed over the buried layer, and has a smaller dopant concentration than the buried layer.
The bipolar transistor has an intrinsic base region of a second conductivity type that is formed on the surface of the first epitaxial layer in the opening. The bipolar transistor also has a layer of isolation material that is formed on the surface of the first epitaxial layer to adjoin the intrinsic base region.
In addition, the bipolar transistor has a first spacer that is formed on the layer of isolation material and the intrinsic base region, and a second spacer that is formed on the layer of isolation material and the intrinsic base region. The second spacer is spaced apart from the first spacer. The transistor further includes an extrinsic base that is formed on the layer of isolation material, an intrinsic emitter region that is formed in the intrinsic base region, and an extrinsic emitter that is formed on the layer of isolation material.
The bipolar transistor also includes a first conductive spacer that is formed on the first isolating spacer to contact the extrinsic base and the intrinsic base region, and a second conductive spacer that is formed on the second isolating spacer to contact the extrinsic emitter and the intrinsic emitter region of the intrinsic base region. The second conductive spacer is spaced apart from the first conductive spacer.
The present invention also includes a method for forming a low-power high-frequency bipolar transistor. The bipolar transistor is formed on a wafer that has a buried layer and a first epitaxial layer of a first conductivity type. The first epitaxial layer is formed over the buried layer and has a smaller dopant concentration than the buried layer.
The bipolar transistor additionally has an intrinsic base region of a second conductivity type that is formed on only a portion of the first epitaxial layer, and a layer of isolation material that contacts the first epitaxial layer and the intrinsic base region. The bipolar transistor further has an extrinsic base that is formed on the layer of isolation material, and an extrinsic emitter spaced apart from the extrinsic base that is formed on the layer of isolation material. The bipolar transistor additionally has a base spacer that is connected to the intrinsic base and the extrinsic base, and an emitter spacer that is connected to the intrinsic base and the extrinsic emitter.
The present invention also includes a method for forming a low-power high-frequency bipolar transistor. The bipolar transistor is formed on a wafer that has a buried layer and a first epitaxial layer of a first conductivity type. The first epitaxial layer is formed over the buried layer and has a smaller dopant concentration than the buried layer.
The method of the present invention begins by forming a layer of isolation material on the first epitaxial layer, and forming an extrinsic base and an extrinsic emitter on the layer of isolation material. The extrinsic base, which is spaced apart from the extrinsic emitter, has a second conductivity type while the extrinsic emitter has the first conductivity type.
The method also includes the step of etching the layer of isolation material to form a first opening in the layer of isolation material. The first opening is between the extrinsic base and the extrinsic emitter, and exposes a surface of the first epitaxial layer. The method further includes the step of forming an intrinsic base region on the first epitaxial layer in the first opening.
The method additionally includes the step of forming a first insulating spacer, a second insulating spacer, and an insulating plug on the intrinsic base region in the first opening. The first insulating spacer contacts the extrinsic base, the second insulating spacer is spaced apart from the first insulating spacer and contacts the extrinsic emitter. The insulating plug is spaced apart from the first and second insulating spacers, and formed between the first and second insulating spacers.
Further, the method includes the step of forming a base spacer that contacts the extrinsic base and the intrinsic base between the first insulating spacer and the insulating plug, and an emitter spacer that contacts the extrinsic emitter and the intrinsic base between the second insulating spacer and the insulating plug.
A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings that set forth an illustrative embodiment in which the principles of the invention are utilized.